Vacuume Casting with Bonded Sand, Is it Possible?

[ELM3060]

Hi all and another dumb question from me.

When I start casting, and the main reason I am going to try, is that I need to cast some gearbox casings. Now I have read how thin castings can be difficult, specially when they start to get some size to them.
The largest gearbox case that I will need to cast, will be (sizes are approximates) L-330mm x W-130mm x D-180mm with a wall thickness of 5mm. It has a large bottom (surface area), rising up on all four sides to the 180mm depth where it will have a mating face for the second half of the gearbox. There will also need to be a few vertical internal walls and two steel pins (molded in with support moldings) on the outside side walls for mounting pins. In the base of the gearbox there will need to be quite a bit of extra depth (approximately 60mm) as it will have two bearing seats (1 internal/1 external) to allow for a drive shaft. Possibly, two of the vertical walls will also extend up from this thickened area (which may help in the casting process).

Now I know this would normally be done by die casting, but I was wondering if it could be done by vacuum casting? Has anyone tried to vacuum cast a project this size?

My thoughts were to first use oil bonded sand as I believe?
(1) it has a better finish than green sand?
(2) Its structural strength as a mold, is stronger?
(3)It will not dry out due to vacuum suction (unlike green sand where the vacuum could suck the water from the sand, oil would be less prone to move)?

I was thinking a vacuum box could be made, where the sand bonded mold could be contained inside with only the downsprue and the vacuum port exiting out through the top. I also thought, if the mold is heated, it would allow the alloy to flow throughout the mold better, and to do this, electric elements could be added or I could seal an old electric oven for the vacuum box. This would allow me to heat the mold first, turn on the vacuum, then pour the aluminum into the mold.

If the sand bonded mold was made with the bottom of the case at the top, then the deep section needed for the bearing seats could be used as the downsprue which would hold a large volume of alloy and could feed any shrinkage. This area has to be machined (milled) out anyway for the bearing seats). In the bottom of the mold (which is the mating edge of the gearbox), small vacuum ports made with wire around the models edge, could be made to allow the vacuum to suck the alloy down into its outer extremities, and where solidification would then block the hole once the area is filled.


The only other way I can see a way to make these cases, is to centrifuge the mold, and this may be a better way to get the alloy out to the longer ends where the vacuum may not do it.

I hope I have supplied enough information, to allow your opinions and expertise to start to flow.

Please share your thoughts
Regards
Ed

[Me2]

Have you actually tried casting with the oil bond sand?
What grade of aluminum are you using to cast with? A grade with a
high silicon content will flow easily and fill thin voids quite well.
As you mentioned feed with risers where required.
Try and keep the metal temp below 800 Deg C and degas before you pour.
I have cast long thin fan blades around 2mm thick with no real dramas.

Cam

[ELM3060]

Have you actually tried casting with the oil bond sand?
What grade of aluminum are you using to cast with? A grade with a
high silicon content will flow easily and fill thin voids quite well.
As you mentioned feed with risers where required.
Try and keep the metal temp below 800 Deg C and degas before you pour.
I have cast long thin fan blades around 2mm thick with no real dramas.

Cam

Hi and thanks for the reply me2. No I have not cast at all, that's why I said "When I start casting, and the main reason I am going to try, is that I need to cast some gearbox casings."

Almost everything I have read, has lead me to believe thin casting is very hard, so your fan blades and comments do add some light and hope.

Just how long are your fan blades and how do you pour them, standing vertical or are they laying horizontal? I am guessing Vertical. Any pics?

What percentage would you call a high content (silica)?

Cheers Ed